Song server: search and download songs, mobile web UI

- Sources: "free" (Creative Commons songs with media from
  github.com/UltraStar-Deluxe/songs) and "usdb" (usdb.animux.de with an
  account; audio/video via yt-dlp from the song's #VIDEO hints or YouTube
  links in the comments, covers from fanart.tv/YouTube)
- Aggregated search (all sources in parallel, in-library flag), download
  queue into <library>/Downloads/<source>/<Artist - Title>, then rescan
- Web UI in wwwroot (plain JS, no build): search, downloads with progress,
  library; phone first, dark mode, installable
- Docker image with yt-dlp, ffmpeg and deno; library mounted writable
- Tests for matching, USDB parsing and song rewriting

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

VocaluxeSongServer/Dockerfile

@@ -8,11 +8,22 @@ COPY VocaluxeSongServer/ VocaluxeSongServer/
 RUN dotnet publish VocaluxeSongServer/VocaluxeSongServer.csproj -c ReleaseLinux --no-restore -o /app
 
 FROM mcr.microsoft.com/dotnet/aspnet:10.0
+# Song downloads (USDB source): yt-dlp for audio/video, ffmpeg for merging, deno as the JavaScript runtime
+# yt-dlp needs for YouTube. yt-dlp is the newest release at build time; rebuild the image to update it.
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends ffmpeg ca-certificates curl unzip \
+ && curl -fsSL -o /usr/local/bin/yt-dlp https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp_linux \
+ && chmod 755 /usr/local/bin/yt-dlp \
+ && curl -fsSL -o /tmp/deno.zip https://github.com/denoland/deno/releases/latest/download/deno-x86_64-unknown-linux-gnu.zip \
+ && unzip -q /tmp/deno.zip -d /usr/local/bin && chmod 755 /usr/local/bin/deno && rm /tmp/deno.zip \
+ && apt-get purge -y unzip && apt-get autoremove -y && rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY --from=build /app .
-# The song library (read-only is enough); see README.md
+# The song library; must be writable for downloads (new songs go to <library>/Downloads)
 VOLUME /songs
-ENV Library__Path=/songs
+ENV Library__Path=/songs \
+    HOME=/tmp \
+    XDG_CACHE_HOME=/tmp/cache
 EXPOSE 8080
-USER $APP_UID
+# Runs as the user given with --user (on Unraid 99:100 = nobody:users, the owner of the shares)
 ENTRYPOINT ["dotnet", "VocaluxeSongServer.dll"]